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Rocky Mountain Iris

Iris missouriensis

by Kathy Lloyd, Montana Native Plant Society

Photo of Rocky Mountain Iris. Rocky Mountain Iris

Rocky Mountain iris or Missouri iris (Iris missouriensis) was collected on July 5 or 6, 1806 by Captain Lewis on the return trip through Montana. On July 3 rd Lewis and his party separated from Captain Clark at Traveler's Rest near Lolo Creek in western Montana. Lewis was enroute to the upper reaches of the Marias River, while Clark headed south to retrieve the items left in a cache at Camp Fortunate near the junction of the Beaverhead River and Horse Prairie Creek. Clark and his party, which included Sacagawea and her baby, Jean Baptiste, or Little Pomp as members of the expedition called him, were to head down the Yellowstone River to meet Lewis at the confluence of the Missouri and Yellowstone Rivers. Part of Clark's party, led by Sergeant John Ordway, planned to travel down the Missouri River from Three Forks to meet part of Captain Lewis's party below the Great Falls with the boats. It was the only time the expedition had split up and three separate parties made a considerable dent in the strength of the expedition and caused the captains considerable worry; but to meet their objectives it was the only course of action they could see to follow.

Captain Lewis wanted to explore the Marias River to see if it reached as far north as the 50 th Parallel, which would have allowed the United States to claim more land in what is now Canada. As Lewis and his party passed through the "prarie of the knobs," as Lewis referred to the area we know as the Ovando Valley, Lewis made a number of plant collections. One of them was Rocky Mountain iris and Lewis's original collection can be seen today at the Academy of Natural Sciences in Philadelphia.

On July 6, 1806 Lewis reported in his journal that he, "saw the common small blue flag and peppergrass. the southern wood and two other speceis of shrub are common in the prarie of knobs. preserved speciemines of them." That comment from Lewis reminds me to tell you that the original spelling from the journals is maintained in these plant abstracts. Spelling was more creative in those days! Frederick Pursh, a botanist of the day who ended up with some of the Lewis and Clark plant collections, wrote on the label, "A pale blue Species of Flag. Prairi of the Knobs Jul. 5 th 1806."

Rocky Mountain iris, a member of the iris family, Iridaceae, is a perennial with sword-like blades for leaves. The flowers are pale blue to light lavender with yellow patches and purple veins. The flowers are large and showy and have three sepals, three petals and three styles and the plants can reach a height of two feet. The plant is rhizomatous and will spread from underground stems, often forming large colonies when conditions are right. Rocky Mountain iris grows in open meadows that are wet in the spring or in other areas with some moisture. The species can be found from British Columbia to New Mexico and east to Minnesota. It is considered rare in Alberta, North Dakota and Nebraska. This iris is the only native iris found in Montana. It is fairly common in western and central Montana and makes a good choice as a native landscaping plant. It is fairly easy to grow from seed and should be planted in areas that receive some moisture.

As you admire our Rocky Mountain iris this spring, remember that a collection made in Montana in 1806 made it all the way down the Missouri and Mississippi rivers to St. Louis and had a number of adventures of its own before finally being housed in the Lewis & Clark Herbarium in Philadelphia.